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Q:
Very fast idle

asked by
Tom H
on
August 01, 2016

This is a very old van and we have had difficulty with the rough engine idle, but now is is Idling very fast. , I have replaced the throttle position sensor, the idle air control and throttle body gasket, cleaned out everything in and around the throttle body. I used an unlit propane torch to look for vacuum leaks, but none could be found. It drives ok but the idle is the crazy part. Any suggestions. Thanks in advance, Tom

Enter your car for a more accurate estimate

Using the propane torch is indeed a good method to find vacuum leaks, but it is by no means the easiest method to use. It takes practice to get good at using it, and often a vacuum leak is just out of reach of what adding external propane can do. One place in particular is not accessible and that is the PCV valve. If it is stuck open, you will have a constant vacuum leak. Another component is the brake booster. It has a check valve in it mounted on the booster. If the booster has a leak, you would have harder than normal breaking and a vacuum leak.

These are just two possibilities, I could probably come up with a few more. The absolute best method to find a vacuum leak is to use a smoke machine. And of course, you can use your ear to follow the hiss. This is often effective, but only with larger leaks.

We are assuming that there is a vacuum leak. It is possible that the PCM isn’t controlling the IAC at all and it is stuck. Testing this is best done with a scan tool that is capable of activating the IAC. If this is the case, I would expect to see a Check Engine Light, but not always.

Aside from that, still assuming you have a vacuum leak, grab a pair of long pliers and pinch off any rubber vacuum hoses one at a time and see if the idle adjusts to normal, or if the pinching off makes a difference at all. This would be a clue to something that maybe going on.

If none of this helps you find the source of your high idle, or you need help performing tests, a certified technician from YourMechanic can come to your car’s location to diagnose your vehicle’s idling issue.

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